The present invention relates to an optical information reading apparatus, and more particularly to an optical information reading apparatus having a servo system for maintaining the perpendicularity between the optical axis of a light beam used for reading recorded information and the plane of the recording medium.
If the angle between the optical axis of a light beam used for reading recorded information and the plane of a recording disk forming the recording medium is offset from a right angle, information can "leak" from an adjacent recorded track to produce cross talk. There are various reasons why this angle may become offset from a right angle. For example, offset may be caused because of deformation of the recorded disk due to aging or because of a slant of the axis around which the disk is rotated due to deformation of the deck portion of the reproducing apparatus. In any event, the generation of cross talk cannot be avoided by the manufacturer of the reproducing apparatus because all of the causes therefore arise after the product has been shipped.
Accordingly, there has been provided a servo system for electrically detecting such cross talk to accurately maintain the perpendicularity between the recorded disk and the optical axis of the reading light beam to thereby reduce the amount of cross talk. Such a technique is disclosed in detail in Japanese Laid-Open Patent Application No. 186,237/1982. In this example, the technique is applied to a reproducing apparatus for a recorded disk which has been recorded using a constant linear velocity (CLV) process. Cross talk is detected using the fact that, when cross talk is present, the synchronizing signal sections of the disk are not arranged on the same radial lines over the entire recorded track in the CLV system. (They are arranged on the same radial line over the entire recorded track in a constant angular velocity (CAV) system.) That is, the leakage component of the synchronizing signal information on adjacent tracks in the CLV system is detected to detect the amount of cross talk, and the angular orientation of the optical axis of the information reading light beam is controlled so as to make the amount of cross talk zero.
Such a system has disadvantages, however, in that the circuit for detecting cross talk is complicated and expensive, and in that the system can be applied only to a CLV system and not to a CAV system.
Another technique is disclosed in Japanese Laid-Open Patent application No. 179,954/1982. FIG. 1 shows the outline of the system described therein, which includes an objective lens 3 for focusing a reading light beam having an optical axis 1 onto a recording plane 2. Further, there is provided an auxiliary beam light source 4 for detecting the amount of offset in perpendicularity between the optical axis 1 and the plane 2. The auxiliary beam is directed onto the recording plane 2 through the objective lens 3. The resulting reflected light passes through the objective lens 3 again and strikes a pair of light detecting elements 5 and 6. If the recording plane 2 is perpendicular to the optical axis, the auxiliary beam is reflected symmetrically with respect to the optical axis to return to a position symmetric to the light source 4 with respect to the optical axis 1, as shown by a solid line in FIG. 1. If the recording plane 2 is inclined as shown by the one-dot chain line or broken line, the position of the returned portion of the auxiliary beam is offset in the direction of the one-dot chain line or the broken line.
The outputs of the light-detecting elements 5 and 6 are applied to a differential amplifier 7 which produces a detection output. The detection output is differenced with a reference signal 9 by a subtractor 8 and the result applied to drive an optical axis tilt control mechanism 10 to thereby maintain the perpendicularity between the optical axis and the recording plane.
Due to the presence of the auxiliary beam, this technique has specific drawbacks:
First, since a servo system (hereinafter referred to as a tilt servo) for maintaining the optical axis 1 of the record information reading light beam perpendicular to the recording plane cannot be operated until the focus servo system of the apparatus has stabilized, the operation of the tilt servo system is slow. In extreme cases, neither tilt servo nor focus servo lock-in can be effected. That is, in order to maintain the auxiliary beam parallel to the optical axis 1 and hence to be reflected to correct positions on the light-detecting elements 5 and 6, the disk plane must be at the focus of the lens. The position of the reflected light from the auxiliary beam will be incorrect if the disk plane deviates significantly from the focus plane. In such a case, an erroneous output is produced from the differential amplifier 7, resulting in inaccurate control of the angle of the optical axis 1.
Accordingly, in the case where the optical axis is set at an incorrect angle, the position of the reflected information-bearing beam on a detector (not shown) used for supplying a control signal to the focus servo or the tracking servo changes so that an unwanted dc component is superposed on the error signal used in the servo system.
Further, since the pickup must be small in size, it is difficult to mount the optical system for the tilt servo in the pickup. To do so generally requires the viewing angle of the objective lens to be narrowed.
An object of the present invention is to provide an optical information reading apparatus of a very simple arrangement in which the perpendicularity between the recording plane and the optical axis of the reading light beam can be detected independently of the operation of the focus servo system.